Spark plug



May 15, 1934. A. A. KASARJIAN SPARK PLUG Filed July 17, 1951 1orgklzszzga m, or e fie.

INVENTOR.

Armen A kasa/y/an A TTORNEY.

Patented May 15, 1934 UNITED STATES PATENT OFFICE SPARK PLUG ApplicationJuly 17, 1931, Serial No. 551,318 5 Claims. (Cl. 123-169) This inventionrelates to spark plugs, and the object of the invention is to improvethe spark at the proper gap through the elimination of parasiticdischarges and leakages of the ignition current.

Exhaustive experimentation and research, particularly with aviationplugs, has convinced me that the most serious difficulty encountered inplug operation is the direct result of parasitic discharges, due,primarily, to the presence of air pockets in the inner plug structure.Such pockets will of course vary in size according to the design of theplug or their location therein. In some cases, they may be relativelylarge, while in other cases, they are a mere film of air betweenadjacent parts of the plug. I have found that the greatest difiicultyarises when these pockets are substantially sealed by the plug structureduring assembly of the parts and where such pockets occur betweenmetallic parts of opposite electrical polarity.

Air under ordinary conditions is a fairly good insulator, but it is byno means a perfect insulator particularly with respect to high tensioncurrent, such as is used for ignition purposes. My experiments havedefinitely shown that when high tension current is impressed uponmetallic parts separated by an air gap, there occurs a silent dischargeor corona discharge. If the air is confined within the substantiallyclosed pocket, such as is commonly found in conventional plug assembly,ionization of the air results, and such ionization proceeds rapidlyuntil a point is reached at which the di-electric property of air is solowered that electric current will pass through it in the form of a jumpspark. When this occurs within a plug structure, short circuiting of theproper gap of the plug results.

In the design of a spark plug, the resistance between the electrodes ofthe proper sparking gap of the plug is made very small in comparison tothe resistance to any other path which the current may take between theconductive parts of opposite polarity. These other paths, I will, forthe sake of brevity, hereinafter refer to as parallel paths. This ispurposely done so that the gap will be ionized as rapidly as possibleand broken through by the spark. Electrical discharges in the otherparallel paths are parasitic and harmful to the proper operation of theplug and thus the higher the resistance of these parasitics the moreefiicient the discharge at the proper gap.

It is characteristic, however, of spark plug oper ation that as thespark jumps between the electrodes at the gap proper, the electrodes areeroded or burned away and the gap is gradually increased in dimensionswith consequent increase in resistance. This action of erosion isparticularly noticed where the high test fuels are used or where tetraethyl lead is used in the fuel. Eventually the gradual increase of theresistance at the gap proper reaches a value above the resistance of oneor more of the so-called parallel paths, so that the current-instead ofjumping the gap proper to form the igniting spark, jumps across theparallel path of less resistance and the plug is said to be shortcircuited.

Where air pockets are sealed in the plug during assembly of the plugstructure, the ionization of the air in such pockets materiallydecreases the di-electric property of the air as stated, and,consequently, the di-electric properties of the air in such pocket arerapidly decreasing at this point while the resistance at the proper gapis increasing. In other words, the resistance to the proper path ofcurrent is increasing, while the resistance to the improper path ofcurrent is decreasing. With such conditions prevalent, the plug does notlong remain operative, but becomes short circuited and ceases tofunction. This condition is particularly noticeable in plugs used foraviation purposes, and which plugs generally employ, because of the hightemperatures encountered, mica insulation. Thepurpose of the presentinvention is to eliminate the so-called parallel paths, within the plugstructure which, in the great, majority of'cases, I have found to bedirectly attributable to the presence of air pockets, and consequentsetting up of a so-called silent or corona discharge as hereinbeforedescribed. I have discovered that by eliminating these air pockets, thesilent or corona discharge within the plug structure is entirelyeliminated and the resistance within the plug structure can at no timebecome less than the resistance at the gap. Accordingly, I thuseliminate parasitic discharges and am able to produce a plug that isabsolutely dependable for a much longer time.

The present invention may be practically carried out in various wayswithout departing from my broad inventive concept. However, I find itessential to the carrying out of this invention that the presence of airpockets within the plug structure be eliminated and thus according tothis invention, the presence of air within the plug structure iseliminated by the actual filling of all spaces within the plug structurewith a suitable substance of high (ii-electric value under thetemperatures prevailing in the plug during operation.

Various substances may be employed for the carrying out of thisinvention, but I may mention, for example, silicate or ceramic compoundsor mixtures, plastic mica or the like which may be applied in a plasticor semiliquid condition under pressure to fill all the pockets andcrevices inside the plug. This filling is accomplished during theassembly of the parts of the plug and all air is thereby expelled fromthe interior of the plug structure. I

By preference, the substance or compound referred to is applied inliquid or semiliquid form, and after assembly may be left in liquid formor baked into a solid, insulating substance.

Experience has shown that this invention materially improves theperformance of plugs by eliminating parasitic paths within the plugstructure resulting from the elimination of air and thereby reducing thecorona discharge. The proper gap at the plug may become larger thanheretofore which will give longer life to the spark plug between periodsof overhaul. A better and more efficient spark alsoresults through theelimination of the parasitic discharges in the parallel paths to which Ihave referred, by concentrating all of the energy of the spark generatorat the proper gap.

Features of the invention, other than those specified, will be apparentfrom the hereinafter detailed description and claims, when read inconjunction with the accompanying drawing.

The accompanying drawing illustrates different practical embodiments ofthe invention, but the constructions therein shown are to be understoodas illustrative, only, and not as defining the limits of the invention.

Figure 1 is a central section through a plug of high compression typeand with which the present invention is shown as associated.

Figure 2 is a similar view of a plug of the low compression typeembodying the present in vention.

Inasmuch as the present invention is not restricted to any particularkind or type of plug, I do not consider it necessary to describe ingreat detail all the parts of the plug structure shown in the drawing asthis is, in the main, conventional and illustrative. It is sufiicient tospecifically describe only those parts of the plug as will be necessaryfor a full understanding of this invention.

In Figure 1, 1 designates the central electrode of the plug to whichthe, ignition lead is adapted to be secured in the usual manner. Thecentral electrode carriers the sparking point 2, while the groundedelectrode 3 is carried by the shell 4 of the plug. In the particulartype of plug shown, the central electrode is surrounded for a greaterportion of its length with a conductive sleeve 5, preferably of brass orcopper or some other highly .conductive material. In fabricating theplug, there is wrapped about this sleeve sufficient thicknesses or turnsof cigarette mica 6. In the wrapping of this cigarette mica, one end ofeach piece as it is applied is dipped into the insulating substance, soas to apply thereto a body of the substance and. as the mica is wrappedabout the sleeve 5, there is sufficient insulating substance thereon tospread itself over both surfaces of the mica and over the surface of thesleeve 5, so that each successive turn ofthe mica becomes embedded inthe substance. As a result, when the cigarette mica has been wrappedabout the sleeve 5 as stated, the

. wrapped body becomes in effect a solid body from which all air hasbeen excluded by the flow of the insulating substance. In other words,no air will be entrained in the pockets or in film form betweenconsecutive turns of the cigarette mica. Of course the same result canbe obtained by coating one or both surfaces of the mica beforeapplication, but this has not been found necessary as the substancewhich I have described will fiow freely.

An important advantage in filling the space between the laminations ofthe cigarette mica is that this mica is apt to crack during winding.These cracks are, of course, very fine, but if the insulating substanceis used as stated, all of these minute cracks are filled and possibilityof short circuiting between the parts 11 and 12 is avoided.

In assembling the insulating body '7, which is composed, as shown inFigure l, of laminated disks of mica, these disks are coated-with theinsulating substance in liquid or plastic form, as they are put on, and,consequently, all spaces between consecutive laminations, as well as thespace 8 between the inner periphery of these disks and the cigarettemica body 6 is fiowed full of the insulating substance, as clearlyindicated in Figure 1.

In assembling the insulating body 9, which is also built up of micadisks, these disks are also coated with the insulating substance inliquid or plastic form, so that all spaces between consecutivelaminations or plies, as well as the space 10 between the innercircumference of this body and the cigarette mica body 6 is flowed fullof the insulating substance and thus all air is expelled during thisoperation. It will of course be understood that the parts of the plugstructure are assembled. under pressure, so that the insulatingsubstance is caused to fill up all spaces in the interior of the plugstructure and thereby exclude air therefrom through displacements ofsuch air.

After the plug is fully assembled, the substance may be left in liquidor plastic form, or it may be baked into a solid insulating substance. Ihave referred to the assembling of the cigarette mica about the sleeve 5by dipping it into the insulating substance before wrapping the micaabout the sleeve. The operation is somewhat similar from a mechanicalstandpoint to the practice which has been heretofore employed of dippingcigarette mica into some lubricant, such as lubricating oil or linseedoil, the purpose heretofore being to facilitate the tightening of theplug against gas leakage.

My experience, however, with the use of oil, as stated, is that the oilwhich under such conditions works into the spaces 8 and 10, quicklycarbonizes, so that the spaces 8' and 10 are divided graphicallyspeaking into a series of relatively small gaps interposed betweenbodies of carbon, whereby the resistance to the passage of current isquickly reduced below the resistance of the gap between the electrodes 2and 3 which describes the usefulness of the plug.- The insulatingsubstance of the present invention is necessarily a refractorydi-electric substance, which will not carbonize, but I have foundnevertheless that when using such a substance I obtain all theadvantages inherent in the use of lubricating or linseed oil, so far astheir function of facilitating the tightening of the plug is concerned,and at the same time obtain a permanent di-electric with a permanentexclusion of air. This can never be acccomplished by the oils whichhaveneaaaeo heretofore been used to facilitate tightening of the plug partsduring assembly.

The advantage of this invention as applied to a plug, such as shown inFig. 1, may be made clearly apparent by a brief description of whathappens in a plug of this type when this invention is not incorporated.If it be assumed that the space 10 is free from the insulating substanceof this invention, it will he noted that there is a rather long, narrowannular air space interposed between the conductive material 11 of onepolarity and the conductive material 12 which by virtue of its elecrical connection with the shell- 4 is of oppositr' polarity. Inpractice, air will be entrained in this space 10 during the assembly ofthe plug. As a result, a so-called silent or corona'discharge will indue course result through this body of air between the parts 11 and 12.1 lhe air will shortly be ionized and if the gap between the electrodes2 and 3 increases in resistance materially, it will not be long beforethe spark will be jumping between the parts 10 and 12, instead ofbetween the parts 2 and 3. r In a similar manner, if the space 8 is inthe form of an air space between the part 14 which is grounded withrespect to the shell 4 and the sleeve 5 which is of opposite polarity,it will not belong before current will be jumping from the part 5beneath the lower edge of the cigarette mica body 6, and thence upwardlyto the part 14, due to the ionization of the air as hereinbeforedescribed.

When these spaces 8 and 10 are, however, so

filled that the cigarette mica body 6 and the will nevertheless still bethe point of least resistance to the passage of current between thecentral electrode and the shell. Consequently, a plug embodying thisinvention, will operate longer and with a larger gap than any othersimilar plug now known.

The structure of Figure 2 will'be perfectly clear from the descriptionalready advanced in connection with Fig. 1. It will be noted'that inFig. 2, 1 is the central electrode, 6 the body of cigarette mica, woundin the manner described,

' directly about the central electrode without the interposition of thesleeve 5 and the body of mica 9 corresponds to a similar body of theplug of Figure 1. The space 10 is filled with the insulating substancein the same manner as described with reference to Fig. 1, and thevarious laminations of the body 9 are assembled in like manner, so thatthe cigarette mica body, as well as the insulating body 9, together withthe in sulating substance in the space 10 becomes one solid block ofinsulation, excluding all air from the inner plug structure.

It is important to note that the incorporation of the present inventionin the plug construction does not increase the cost of production otherthan the mere cost of the insulating substance employed which isnegligible, and, nevertheless, through the employment of the presentinven- Having thus fully described the invention, what I claim as newand desire to secure by Letters Patent is:

1. In a spark plug, a central electrode, cigarette mica wrapped aboutsaid electrode in close fitting relation thereto to form a laminatedsleeve of insulating material, a refractory insulating substanceinterposed between said laminations and between the laminated sleeve andsaid electrode to preclude the formation of air pockets betweenconsecutive laminations, an insulating member surrounding the laminatedmica sleeve, and a refractory insulating substance filling the spacebetween said insulating member and the sleeve to preclude the formationof air pockets between the sleeve and said member.

2. A spark plug embodying a shell provided thereon with a groundedelectrode, an insulating element mounted within said shell, a centralelectrode extending through said insulating element and in relativelyclose fitting relationthereto, and means for precluding coronadischarges through the plug comprising an insulating refractorysubstance filling the air pockets within the insulating element toexclude residual air from said element.

3. A spark plug embodying a central electrode, a, spaced groundedelectrode, an insulating element for spacing said electrodes apart whichinsulating element embraces the central electrode in close fittingrelation, and means for preventing corona discharges in the plugcomprising an insulating refractory substance impregnating theinsulating element to an all pockets therein for the purpose ofexcluding air from said insulating element.

. 4. In a spark plug, a central electrode, cigarette mica wrapped aboutsaid central electrode in close fitting proximity to form a laminatedsleeve, and an insulating refractory substance filling all pocketsbetween consecutive laminations of the sleeve and between the sleeve andelectrode to exclude air from the sleeve and from between the sleeve andthe electrode and to thereby prevent corona discharges through saidsleeve.

5. In a spark plug, an insulating element provided with a longitudinalpassage therethrough for the insulating electrode of the plug, aninsulated electrode extending through said pas- I sage in close fittingrelation to the wall thereof,

and an insulating refractory substance impregnating said element to fillall pockets therein for the purpose of excluding residual air to preventcorona discharges.

ARMEN A. KASARJIAN.

